Battery likely not at fault but I would replace it. After 18 min. you have lower voltage than before you started it. Reason being alternator is not keeping up with load requirements. After initial start your voltage looks good then time and rotten connections become concerns to me. First check the ground wire from alternator to engine case behind the fan. If there isn’t one then install one. Next check all ground points. They are grouped together in several places on the car. Main grounds on chassis and fuse box area. Also check main alt. feed wire to starter. If all connections are good then pull the alternator and check brushes/slip rings. High mileage will wear the the slip rings to alarming depths. The slip rings can be changed but you may consider alt. replacement if wear is a concern.
Side story: We did a 1400 mile Hershey trip two years ago and this past March I pulled the engine (‘87 3.2) to do the clutch. I was amazed at how dirty/rotten/loose a number of connections were. Enough so I am now amazed the car went with absolute reliability. Make haste slowly...
